The present invention relates to a rubber composition suitable for use in tire treads, and more particularly to a rubber composition having excellent characteristics on icy and snow-covered roads and suitable for use in treads of studless tires.
Spiked tires which grip icy roads by spike pins, have been popularly used as pneumatic tires suitable for running on icy roads. However, the spiked tires cause dust pollution because roads are scraped off by the pins if automobiles with spiked tires are run on usual roads. The use of spiked tires is in some countries legally prohibited in order to prevent dust pollution from spiked tires. Thus, in recent years, studless tires attract attention as pneumatic tires which can grip icy roads without the use of spike pins. However, the characteristics on icy roads of studless tires are still insufficient as compared with spiked tires which directly grip icy roads by means of the spike pins. Thus, in order to improve the characteristics on icy roads of studless tires, there have been made various attempts to increase the coefficient of friction of the tread surface to icy roads.
Among frictions between a tread and a road, digging friction and adhesive friction are known as main frictions to secure the driving force and braking force. It is necessary to increase these frictions in order to improve the grip characteristics on icy roads of studless tires.
For example, it is proposed to produce the tread of studless tire from an expanded rubber, wherein it is contemplated to increase the digging friction by means of edges resulting from cells of expanded rubber and to increase the adhesive friction by the action of the edges to remove a water film formed between the tread surface and an icy road. Improvement in digging friction and adhesive friction requires increasing the proportion of cells, namely expansion ratio, but this results in softening of the tread, and in its turn brings about deterioration of stability, controllability and abrasion resistance in a non-snowy area.
Japanese Patent Publication Kokai No. 2-167353 proposes a pneumatic tire having a tread made from a rubber composition incorporated with a powdery product of a cellulose material such as chaff or wood flour. Such a cellulose powder does not form any chemical bonding with a rubber component. Thus, the cellulose powder exposed onto the surface of tread will fall off during running, and open pores resulting from falling off of the cellulose powder from the tread may contributes to increase in digging friction and adhesive friction in the same manner as cells of an expanded rubber. On the other hand, since the cellulose powder is present as an additive in the rubber before it is exposed to the tread surface and falls off therefrom, softening of the tread rubber as encountered by the expanded rubber does not occur, thus this tread rubber does not bring about deterioration of the durability in a non-snowy area.
However, the cellulose powder has many OH groups in the cellulose molecule and shows a hydrophilic property, so its hardness decreases due to absorption of water before it is exposed to the tread surface and falls off therefrom. Thus, there is a limit in increasing the digging friction by incorporation of the cellulose powder.
It is an object of the present invention to provide a rubber composition suitable for use in treads of studless tires, which has improved digging friction and adhesive friction without deteriorating the stability and controllability and the durability thereof.
A further object of the present invention is to provide a rubber composition having a good durability such as abrasion resistance and capable of exhibiting excellent grip characteristics even on an icy road.
These and other objects of the present invention will become apparent from the description hereinafter.